NMR property calculations and experimental study of the 1,6‐epoxycarvone and α‐epoxypinene: a comparison of models

Abstract

This work aims at using theoretical calculations of shielding tensors (σ) through different methods [gauge-independent atomic orbital (GIAO), continuous set of gauge transformations (CSGT) and individual gauges for atoms in molecules (IGAIM)] and spin-spin coupling constants J using GIAO method to compare these methods and to corroborate the data obtained with the assignment of all of (1)H and (13)C NMR signals and the relative stereochemistry of the 1,6-epoxycarvone and the α-epoxypinene. All the (1)H and (13)C NMR signals were assigned unequivocally. The stereochemistry for the epoxides is trans and the B3LYP theory level with CSGT and IGAIM methods is the best choice to evaluate theoretical chemical shifts for compounds studied.

1H NMR Spectra. Part 28: Proton chemical shifts and couplings in three‐membered rings. A ring current model for cyclopropane and a novel dihedral angle dependence for 3JHH couplings involving the epoxy proton

“…1S). These values confirm both the assignment and the couplings of reference . It is of interest to note that H 2 has two resolved couplings (4.1 and 1.4 Hz).…”

“…Thus, we give in Table 3 J HH couplings from the epoxy proton to the CH 2 side‐chain for selected compounds. These values were obtained directly from the spectra for compounds 28 , 29 ( this work ) , 30 , and 32 from the 13 C satellites of the epoxide protons for 22 and 23 and from an iterative analysis of the spectrum of 31 . It can be seen that the couplings vary from 0.3 to 10.7 Hz, and it was of interest to see whether these values could be reproduced by DFT calculations or whether a simple Karplus‐type equation would suffice.…”

“…. The chemical shifts for compounds 1 , 2 , 3 , 4 , 6 , 12 , 5 , 9 , 10 , 11 , 13 , 14 , 7 , 8 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24, 25, 26 , 27 , 28 , 29 , 30 , 32 , 33 , 34 , 35 , 36 , 37 were taken from the literature.…”

“…1. The chemical shifts for compounds 1, [7] 2, 3, 4, 6, 12, [28] 5, 9, 10, 11, 13, 14, [29] 7, [9,28] 8, [9] 15, [30] 16, 17, [31] 18, [4,28] 19, [27,28] 20, [28] 21, [27,28] 22, [32] 23, [28,33] 24, 25, 26, 27, [18] 28, 29, [19] 30, [33] 32, 33, 34, [29] 35, [28] 36, [29] 37 [28] were taken from the literature. The FID's of 1,6-epoxycarvone (28) and a-epoxypinene (29) were made available to us by Prof. V. Lacerda Jr.…”

“…An early investigation of the magnetic anisotropy contribution of epoxide rings was due to the study of electrophilic addition to 3-norcarenes by Paquette et al [18] . A very recent study of epoxycarvone and epoxypinene [19] showed that gauge independence of atomic orbital calculations can be of use in stereochemistry studies, and a similar study [20] of the stereochemistry of some SO derivatives of arylthioflavan epoxides used ab initio calculations to study the 1 H chemical shift differences between the isomers. However, to date, there is still to our knowledge no comprehensive investigation of the substituent effects of three-membered rings on the 1 H chemical shifts of neighbouring protons.…”